This invention relates to data communication systems, especially such as are suitable for use as local area computer networks.
In such a network the devices which are to communicate with one another often do so over a communications channel the use of which they share in the time domain. Access to the channel for each device is controlled by some form of interface device coupled between the device and the channel. Communication takes place by packets transmitted by the interface devices according to the particular protocol in use. Though the interface device often adds information of a framing nature, the data for the packet is supplied by the device, which is responsible for the amount of data to be included in the packet. Various examples of interface devices suitable for use when the protocol is of the type known as carrier-sense multiple-access with collision detecticn (CSMA/CD) are to be found in Electronics, Oct. 6, 1982, pages 89 to 105. However, if the device is sending data to a single destination, in accordance for example with a virtual connection already established between the two, the need for the device to be responsible for the amount of data in the packet requires an involvement by the device in low-level details of the communications procedure which we have realised is unnecessary.